1. Field of the Invention
My invention relates to the field of electrical tuned circuits, and more specifically to a circuit in which precision linear tuning is accomplished by varying the value of circuit capacitance.
2. Description of the Prior Art
It is common to provide tuning for a circuit by connecting some form of capacitance either in series or in parallel with some form of inductance, the combination being used as a critical part of the functional circuit. Tuning is accomplished by adjusting either the capacitance or inductance, or both, which alters the resonant frequency of the combination. Practical difficulties arise with this arrangement when the application requires tuning over a very wide frequency range while simultaneously demanding that the frequency be adjusted to high precision. One type of previously known tuning circuit includes a single capacitor which must be controlled with high precision. Exact control might be made possible through use of a physically large capacitor or with a high resolution mechanical system capable of precision movement of the electrodes or dielectric of the capacitor. Precision must often be provided by differential screws, optical encoded shafts or laser position monitors. The physical size of the capacitor or the mechanical precision necessary to tune it is determined by the highest degree of tuning resolution required in the circuit. However, because tuning resolution normally varies over the tuning range, any circuit must be designed such that the poorest resolution achieved is sufficiently good to meet the requirements of the circuit. As a result, the physical size of the capacitor or the mechanical precision of the system must often be unnecessarily large over almost all of its tuning range. An alternative scheme uses two or more independently variable capacitors which provide various degrees of coarse and fine adjustment. The additional mechanical linkages increase cost, size and the complexity of the system. Both of the above approaches are still further complicated if expected vibrations in the intended environment will produce continuous variations in capacitance which exceed the tuning resolution requirements. Yet another alternative is to use a number of separate circuits, each covering a portion of the desired frequency range. This results in duplication and increases cost and complexity of the system.
Any electrical circuit necessarily includes some undesired resistance losses which may degrade circuit performance. In some applications it is desirable to make these resistive losses as low as possible. It is well known that this can be achieved by fabricating the circuit from materials which become superconducting when cooled to temperatures near absolute zero. In particular, nearly lossless AC circuits having extraordinarily high Q's can be obtained. Because of the extreme brittleness exhibited by most superconductors at these temperatures the "supercooling" of electrical circuits leads to problems when flexible lossless electrical connections are required. Circuits have been designed which completely eliminate the need for moving electrical contacts or connectors, however, these circuits have failed to provide a highly desirable linear relationship between the frequency range to be scanned and the capacitor adjustment mechanism.
An important means for providing cooling for superconducting devices is the immersion of the device in a bath of liquid helium. For practical applications efficient long hold time liquid helium dewars are used which are carefully designed to minimize heat flow into the cryogenic environment. Such dewars are capable of holding a supply of helium for four months or longer before a refill is required. In applications which require the use of many cables or mechanical linkages, the heat leak down these connections can quickly exceed the heat leak into the dewar alone and severly limit the hold time of the dewar. In these cases closed cycle refrigerators must be used to extract the heat which leaks down the connections. Use of refrigerators can cause vibrational, electrical and magnetic interference harmful to device performance. Use of this invention to minimize the number of linkages to the device will provide important reductions in the amount of heat leakage and thus significantly extend the hold time or even eliminate the need for a refrigerator.
My invention includes a novel combination of a fixed capacitor in parallel with a variable capacitor, the combination of which when added in series or in parallel with an inductance, results in a tuned circuit in which the resonant frequency is directly proportional to the displacement of the dielectric in the variable capacitor.